Medical balloon

ABSTRACT

Low polymer stress balloons are made by expanding a tube radially while allowing the ends of the tube to move axially in response to the radial expansion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of and claims priority toU.S. application Ser. No. 10/263,225, filed Oct. 2, 2002, which is acontinuation application of and claims priority to U.S. application Ser.No. 09/950,195, filed Sep. 10, 2001, both hereby incorporated byreference.

TECHNICAL FIELD

This invention relates to medical balloons, such as dilation balloonsand catheters using such balloons, and methods of making and using thesame.

BACKGROUND

Medical balloons can be deflated and inflated about their longsupporting devices and placed in bodily conduits to administertreatments, for example, deployment of stents or widening of constrictedpassages during angioplasty, valvuloplasty, or urological procedures.

In angioplasty, for example, coronary angioplasty, a balloon can be usedto treat a stenosis by collapsing the balloon and placing it in a bodilyconduit, e.g., a coronary artery. The balloon is then inflated, e.g., byinjecting a fluid, at a region of the artery that has been narrowed tosuch a degree that blood flow is restricted. Inflating the balloon canexpand the stenosis radially so that the vessel will permit anacceptable rate of blood flow. This procedure can be a successfulalternative, for example, to coronary arterial bypass surgery. Afteruse, the balloon is deflated or collapsed and withdrawn.

Medical balloons can be manufactured by extruding a cylindrical tube ofpolymer and then pressurizing the tube while heating to expand the tubeinto the shape of a balloon. The balloon can be fastened around theexterior of a hollow catheter shaft to form a balloon catheter. Thehollow interior of the balloon is in fluid communication with the hollowinterior of the shaft. The shaft may be used to provide a fluid supplyfor inflating the balloon or a vacuum for deflating the balloon.

It is important that the balloon have a generally predictable shape oninflation. Typically, the balloon, such as a regular balloon, shouldhave proximal and distal taper regions with closely matched taper anglesand a uniformly cylindrical dilatation region. A deformed regularballoon, however, may have an irregular profile, such as a taperextending along the length of the dilatation region or portions withnon-uniform cross sectional diameters. As a result, during use, deformedregular balloons may undesirably provide unpredictable, and thusunreliable, inflations/deflations or stent deployments. It is believedthat deformed balloon shapes can be caused by locking the polymericchains in undesirable configurations during manufacture, referred to aspolymeric stress. The release or partial release of this stress, e.g.,during heat sterilization, can also cause deformation.

SUMMARY

This invention relates to medical balloons, such as dilation balloonsand catheters using such balloons, and methods of making and using thesame.

In one aspect, the invention features a method of making medicalballoons that relieves and minimizes stress in the balloon. Theinvention also features balloon devices such as, for example, ballooncatheters and stent deployment systems.

In another aspect, the invention features a method of making a medicalballoon including providing a polymer tube having two ends and suitablefor being formed into the medical balloon, and expanding the tuberadially while allowing the ends of the tube to move axially in responseto said radial expansion. The method can further include axiallyorienting a material of the tube, e.g., by axially drawing the tube.Axially orienting the material can further include internallypressurizing the tube.

In another aspect, the invention features a method of making a medicalballoon including providing a polymer tube having two ends and suitablefor being formed into the medical balloon, drawing the tube axially, andexpanding the tube radially while allowing the ends of the tube to moveaxially in response to said radial expansion.

Embodiments of aspects of the invention may include one or more of thefollowing features. The ends of the tube are allowed to move freely asthe balloon is formed. In some embodiments, the ends of the tube movesinwardly, for example, a distance between about 1 and about 40 percent,e.g., between about 5 and about 30 percent of an original length of thetube, between about 10 and about 25 percent, or between about 15 andabout 22 percent. The tube is below a glass transition temperature of amaterial of the tube during drawing, e.g., below about 100° C. The tubeis internally pressurized during drawing.

Expanding the tube can include heating the tube, e.g., to a temperatureabout equal to or greater than a glass transition temperature of amaterial of the tube. The tube can be expanded to an expanded diameterof the balloon being made. In certain embodiments, the tube can beexpanded to an expanded diameter about 1 to about 15 times, e.g., about5 to about 9 times, larger than an unexpanded diameter of the tube,e.g., by introducing a gas into the tube. Expanding the tube can includesimultaneously heating and pressurizing the tube. Expanding the tube caninclude positioning the tube into a mold, and simultaneously heating andpressurizing the tube. Expanding the tube can include contacting thetube with a liquid, e.g., one including water.

The tube can be formed of a polymer having regions of differenthardness. The tube can include a block copolymer having hard segmentsand soft segments. The block copolymer can be a polyether-esterelastomer, a polyester elastomer, or a polyether block amide. The tubecan include a material such as a polyester and a polyamide. The tube canbe a multilayer tube, such as a coextruded tube having layers ofdifferent hardness. The tube can be formed of a polymer that is not ablock copolymer.

The method can further include characterizing the balloon bybirefringence.

The method can further include attaching the balloon to a catheter toform a balloon catheter device. The method can further includesterilizing the balloon, e.g., by heating the balloon between about 32°C. and about 60° C.

The method can further include attaching the balloon to a catheter toform a balloon catheter device and/or positioning a stent on theballoon.

In another aspect, the invention features medical balloon product madeaccording to the above methods.

Embodiments of the invention include one or more of the followingfeatures. The medical balloon can include a block copolymer having acylindrically-shaped region exhibiting a birefringence pattern ofsubstantially parallel lines before and after an exposure tosterilization temperatures. In certain embodiments, the balloon can beexposed to about 32° C. to about 60° C. The cylindrically-shaped regioncan have a length between about 1.5 cm and about 14 cm and/or an outerdiameter between about 2 mm and about 30 mm, e.g., about 2 mm and about20 mm, or about 2 mm and about 12 mm. The polymer can have regions ofdifferent hardness, e.g., hard segments and soft segments. The blockcopolymer can include a material such as, for example, a polyether-esterelastomer, a polyester elastomer, a polyether block amide, a polyester,or a polyamide. The balloon can be formed of multiple layers, e.g.,multiple coextruded layers having different or the same hardness.

Embodiments may have one or more of the following advantages. Duringballoon formation, the balloon is not constrained in movement or axiallydrawn. Instead, the balloon is allowed to recover or relax, e.g.,contract and/or expand axially and radially. As a result, the balloonscan be formed uniformly and with a minimal level of residual stress.Subsequently, the balloons can then be exposed to a post-formation heattreatment, such as sterilization at temperatures of about 32° C. toabout 60° C., without incurring substantial irregularities ordeformations. The balloons can be produced consistently and predictably,thereby providing reliable results during use. The balloons may also bemechanically stable and have enhanced properties, such as burst strengthand hoop strength. The method can also be performed without the need forcertain production steps, such as annealing, thereby reducingmanufacturing cost and time. The process can be useful for formingrelatively large size balloons. The process can be useful for formingballoons about 1.5-14 cm or more in length and/or about 1-30 mm, e.g.,about 1-20 mm, or about 1-12 mm, or more in diameter, where smallamounts of residual polymeric stress may be manifested as relativelylarge physical deformations. Other balloon sizes are possible.

Other features and advantages of the invention will be apparent from thedescription of the preferred embodiments thereof and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic of an embodiment of a method of making a medicalballoon;

FIG. 2A is a photograph of a medical balloon having a substantiallystraight birefringence pattern;

FIG. 2B is a photograph of medical balloons having a “bump”birefringence pattern and a “bowtie” birefringence pattern;

FIG. 2C is a photograph of medical balloons having a proximal to distaltaper birefringence pattern and a distal to proximal taper birefringencepattern;

FIGS. 3A-3B are schematic views of an embodiment of a drawing machineduring use;

FIGS. 4A-4C are schematic views of an embodiment of a blow-moldingassembly during use;

FIG. 5 is a front view of an embodiment of a gripper assembly;

FIG. 6 is a top view of the gripper assembly of FIG. 5;

FIG. 7 is an end view of the gripper assembly of FIG. 5;

FIG. 8 is an exploded perspective view of an embodiment of a gripper;

FIG. 9 is a plot of applied internal pressure (psi) vs. time (sec)during axial drawing of a 6-4 balloon; and

FIG. 10 is a plot of parison internal temperature (Celsius) vs. time(second) and pressure (psi) vs. time (second) during fabrication of a6-4 medical balloon; and

FIG. 11 is a plot of parison internal temperature (Celsius) vs. time(second) and pressure (psi) vs. time (second) during fabrication of an8-4 medical balloon.

DETAILED DESCRIPTION

Referring to FIG. 1, a method of fabricating a medical balloon 20includes the steps of drawing a tube 22 axially to form a parison 28having a stretched portion 24, and forming the balloon in the stretchedportion. During formation of balloon 20, both ends 26 of parison 28 areallowed to move axially, e.g., contract in the direction of the balloon(shown by arrows 27). The balloon exhibits no or relatively smallundesirable deformation, both after formation and after a post-formationheat treatment, such as heat sterilization in ethylene oxide.Consequently, the balloon provides enhanced performance. In certainembodiments, the balloon exhibits accurate sizing, good abrasionresistance, and/or consistent deflations and inflations.

The amount of residual stress retained by a balloon can be measured byobserving the birefringence of polarized light passing through theballoon. Referring to FIG. 2A, one example of a balloon in which stresshas been substantially relieved can exhibit a birefringence pattern ofgenerally parallel lines 23 across the dilatation portion, or body, 21of the balloon and generally symmetric taper birefringence. Referring toFIGS. 2B and 2C, a balloon in which stress is retained can showwavy-line birefringence 25, asymmetric split line birefringence 27, andbirefringence lines 29 that are not parallel with the cylindricalsurfaces of the balloon. Upon release of this stress, e.g., by heatsterilization, the balloon may deform unpredictably into an undesiredshape.

Without wishing to be bound by theory, it is believed that the deformedshapes in medical balloons can be the result of residual stressesimposed in the polymer during manufacture, e.g., during blowing andheating. Irregular balloon shapes can be particularly problematic whenthe balloons are formed of a material having molecular portions orsegments capable of having different physical properties.

For example, a block copolymer can include molecular segments that arerelatively hard or rigid and other segments that are softer or moreelastic. The rigidity of the hard regions is believed to be caused bythe intermolecular interaction of the polymer chains in these regions,which lock the segments in a particular orientation. In the softsegments, on the other hand, the molecular chains are more mobile, e.g.,they can expand or contract. The sections can distribute themselvesdifferently in response to stretching and temperature changes.

In forming a balloon, a portion of a tube of block copolymer is heated,typically above the glass transition temperature (T_(g)) of the tubematerial, and the polymer is stretched, e.g., radially, by blowing. Thisprocess orients the polymer. As the polymer is cooled, the chains becomelocked in given orientations.

In the present technique, the ends of the polymer tube are free tocontract, expand, etc. as the inflated portion is formed so that thepolymeric chains are able to orient in desirable configurations, thusreducing polymeric stress. In addition, in some embodiments, it ispreferred that the tube is axially stretched and plastically deformed atlow temperatures prior to blowing. The low temperature stretching isbelieved to axially orient the polymer. By carrying this step out at lowtemperatures, e.g., less than T_(g), the polymer orientation is notmodified by heat-induced phenomena. As a result, the balloon isbiaxially oriented with reduced stress.

Balloon 20 can be formed of a resilient material, such as a polymer,that is capable of being inflated and deflated in a patient. Preferably,the material is relatively stable upon crystallization. As discussedabove, the method is particularly applicable to polymers or polymercombinations in which stress produced during manufacture is subsequentlyreleased, e.g., during heat sterilization. Deformation problems can bemost severe when soft polymers are used in combination with hard orrigid, highly crystalline polymers, such as polyethylene terephthalate(PET). For example, block copolymers having soft and rigid blocks orcombinations of different polymers in coextruded layers having differentstress release characteristics can be effectively manufactured withreduced deformations. Examples of suitable materials include blockcopolymers such as a polyether-ester elastomer (e.g., Amitel® EM740,from DSM Engineering Plastics, Evansville, Ind.), a thermoplasticpolyester elastomer resin (e.g., Hytrel® from E.I. du Pont de Nemoursand Co.), or a polyether block amide (e.g., Pebax® from Atofina). Othermaterials can also be used, for example, organic or synthetic polymerssuch as polyimides, nylons, rubbers, latex, or engineered resins. Insome embodiments, the material can have a hardness, measured in Shore Dhardness, between about 50 and about 75.

Referring to FIGS. 3A and 3B, in one embodiment, parison 28 may beformed in a drawing machine 30. Drawing machine 30 stretches tube 22,which axially orients the polymer. The stretching preferably is donebelow the glass transition temperature of the polymer, e.g., at roomtemperature (typically about 15 to about 30° C.), so the parison is notexposed to high temperatures during stretching. Drawing machine 30includes a pair of opposed gripping assemblies 38, 40 between which tube22 can be stretched. Gripping assembly 38 is fixed, e.g., using abolting arrangement 44, to a support table 46 and includes a pressuredtube gripper 48 that clamps a sealed end of tube 22. Gripping assembly40 is moveable. Assembly 40 can be translated along table 46 (arrow 50)by coupling the assembly to a translation device 52, such as a planetarygear coupled to a servo motor. Moveable gripping assembly 40 includes apressurized gripper 58 that grips an open end of the tube 22 and permitsa coupling 60 for introduction of pressurized fluid, such as a gas, froma fluid source into the tube. Suitable grippers are available from, forexample, SMC Corp., Indianapolis Ind. (Model MGQ2-25S). The servo motorand gas source may be interfaced to a computer to draw an end of tube 22a predetermined distance at a predetermined rate while providing apredetermined pressure profile. Multiple sets of gripper assemblies maybe arranged in adjacent rows to simultaneously stretch multiple tubes.Parison 28 may also be stretched by translating both ends 26.Pressurized gas may be introduced from both ends 26.

Tube 22 is generally a cylindrical member dimensioned to be suitable forbeing fabricated into a medical balloon. For example, tube 22 can have alength of about 5 cm to about 42 cm; an inner diameter of about 0.2 mmto about 3 mm; and an outer diameter of about 1 mm to about 4 mm,depending on the size of the balloon to be fabricated and the materialof the tube. Other tube sizes can be used.

The dimensions of stretched portion 24, e.g., length, and end tapers,are a function of the drawing parameters, such as the rate of drawing,the distance of drawing, the initial dimensions of tube 22, the materialof the tube, the internal pressure, inner diameter, and outer diameter.Generally, the larger the balloon to be fabricated, the more tube 22 isdrawn and the longer the stretched portion. The rate of drawing can berelatively slow or relatively fast, e.g., about 0.02 cm/sec to about 0.6cm/sec. Faster draw rates can improve production throughput.

The rate and distance of drawing are generally balanced with the appliedinternal pressure to form a substantially uniform stretched portion 24.Relatively low pressure is typically applied at relatively slow drawrates to avoid bulging stretched portion 24 and/or bursting tube 22.Conversely, at relatively high draw rates, relatively high pressure isapplied to maintain the integrity of the lumen of tube 22 and tominimize crystallization in stretched portion 24, which could laterappear as defects during balloon formation.

Referring to FIGS. 4A-4C, in one embodiment, parison 28 is formed into aballoon by expanding stretched portion 24 under pressure while heating.During expansion, both ends of parison 28 can contract axially torelieve stress introduced during the axial stretching or radialexpansion. In one embodiment, balloon 20 can be formed using ablow-molding assembly 70, which includes a heated mold 72 and twoopposing gripper assemblies 74, 76 that are translateably mounted on asupport table 78 using low friction couplings 80, 82. The mass of thegrippers and the low friction couplings 80, 82 are arranged to allow theparison to move freely or with minimal inhibition. In some embodiments,the mass of the grippers and the low friction couplings 80, 82 arearranged to provide a small frictional resistance, e.g., having acoefficient of friction of about 0.003, to axial motion compared to theforce of the relaxing polymer. Suitable couplings include ball slides.In other embodiments, the ends of the balloon can be mechanically urgedinwardly, e.g., using a motor to release stress.

Mold 72 may be a clamshell-type mold having an upper portion 84 and alower portion 86 that can be positioned about stretched portion 24, andincludes a mold cavity 85 of desired shape. Mold 72 can be made of anymaterial that is stable, e.g., does not melt, at balloon formingtemperatures and that can be heated, e.g., by an infrared lamp or by aresistive or inductive heater. Suitable materials include those withrelatively high thermal conductivity such as, e.g., aluminum, stainlesssteel, beryllium copper alloys, and glass.

Gripper assemblies 74, 76 include pressurized grippers to grip the endsof the parison and couplings 79 to permit the introduction ofpressurized gas. Throughout balloon formation, pressure is applied inthe parison. The amount of applied pressure depends, for example, on thesize of the balloon and the properties of the parison such as hoopstress. For example, larger balloons tend to require less pressure. Toomuch pressure can burst the balloon, while too little pressure canproduce incomplete balloon formation.

To form balloon 20, parison 28 is positioned between gripper assemblies74, 76 and mold 72 is positioned about the central portion of theparison (FIG. 4A). Mold 72 is then heated, typically above the glasstransition temperature of the parison polymer(s), while pressurized gasis introduced into the parison lumen, causing the central portion of theparison to expand into the shape of the mold (FIG. 4B). As evident, theends of the parison engaged by the grippers are not substantiallyheated. As the temperature of the mold is increased and the polymerheats above T_(g) and is pressurized, stresses induced during blowmolding and axial stretching are allowed to relax by permitting the endsof the parison to be drawn inward (arrows 88, 90) (FIG. 4C). The parisoncan axially relax between about 5 to about 30 percent of its length. Theends of the parison move inwardly during blowing since the grippers aremounted on low friction couplings 80, 82, which are slideable inwards.The stress release may be gradual as the balloon forms. Typically, theballoon expands to a diameter of about 5-9 times the inner diameter oftube 22. After reaching the desired diameter, mold 72 is cooled.

Referring again to FIG. 1, after formation of balloon 20, the balloon iscooled, the end portions are cut away, e.g., the portions extendingoutwardly from the smallest diameter of the parison, and the balloon isassembled upon a suitable catheter 92 that has a balloon inflation lumen94 for inflation of the balloon and a through lumen 96 for receiving aguidewire. Radiopaque markers 98 can be provided on catheter 92 near theends of the main body of balloon 20. Catheter 92, including balloon 20,can then be sterilized.

In other embodiments, balloon 20 can be used with a balloon expandablestent to form a stent delivery system. For example, the stent is crimpedto its reduced diameter over a delivery catheter, positioned at adeployment site, and then expanded in diameter by fluid inflation of theballoon positioned between the stent and the delivery catheter.

In other embodiments, the step of axially drawing tube 22 can beeliminated. For example, tube 22 can be pre-formed axially orientedand/or include a segment having a diameter equal to stretched portion24. Alternatively, a hypotube having an outer diameter equal to theinner diameter of stretched portion 24, can be inserted into tube 22prior to axial stretching to maintain the lumen of the stretchedportion.

In other embodiments, during axial drawing, both ends of tube 22 can bedrawn. During balloon formation, the pressuring fluid can be introducedthrough only one end of parison 28. Other balloon formation techniquesmay be used. For example, free blowing without a mold may be used.

In some embodiments, mold 72 includes an indicator, e.g., a hash mark,that can be used to place parison 28 in the mold at a predeterminedposition. The indicator can minimize systematic error.

In other embodiments, the methods described above can be applied toother balloon configurations, such as, for example, step, ornon-regular, balloons.

Further embodiments are illustrated in the following examples, which arenot intended to be limiting.

EXAMPLE 1

FIGS. 5-7 illustrate a particular gripper assembly and low frictioncoupling.

Each gripper assembly 100 includes a gripper 102 that is connected,e.g., by brackets 104, to a vertically-mounted ball slide, or crossedroller slide, 106 and a horizontally-mounted ball slide 108. Eachgripper 102 connects to one end of a parison and allows a pressurizinggas to be introduced into the lumen of the parison. Slides 106 and 108allow the parison to move freely, e.g., contract axially and/or movevertically within a mold, during balloon formation. Slides 106 and 108are commercially available from, for example, PIC Design Corp.,Middlebury, Conn. (Part No. PNBT-1080A).

Referring to FIG. 8, gripper 102 includes a cube-like housing 110. On afirst face of housing 110, gripper 102 receives a silicone grommet 112and a grommet cap 114. On a second face opposite the first face, housing110 receives a balloon guide 116 having an opening 118 through which theparison is fitted. On a third face, housing 110 receives a piston 120,including O-rings 122, and a piston cap 124. Generally, gripper 102mates with the ends of the parison by applying pressure to piston 120 tolock the parison in place. Simultaneously, pressure is applied aroundgrommet 112 to allow the inflation pressure to flow into the parisonwithout leaking.

EXAMPLE 2

The following illustrates formation of a 6-4 (i.e., 6 mm O.D. and 4 cmlong) Amitel® (EM740 from DSM Engineering Plastics, Evansville, Ind.)balloon.

A tube of Amitel® was provided having an inner diameter of about 0.086cm (0.034 in), an outer diameter of about 0.177 cm (0.07 in), and alength of about 30.48 cm (12 in). The tube was sealed on one end byheating and pressing the end with pliers. The tube was formed into aparison by axially drawing the open end of the tube at room temperature.The draw rate was about 0.41 cm/sec (0.16 in/sec); the draw length wasabout 10.16 cm (about 4 in); and the maximum internal nitrogen pressurewas about 450 psi (FIG. 9). A stretched portion about 10.16 cm (about 4in) long and having an O.D. of about 0.152 cm (0.06 in) was produced.The sealed end of the tube was removed.

The parison was formed into a balloon using an aluminum clamshell moldpositioned between a gripper assembly described in Example 1. The lowerhalf shell included a thermocouple positioned at the center to monitorthe temperature of the mold during use.

Before forming the balloon, the lower half of the mold was sprayed witha liquid, such as water, until the lower half was uniformly coated. Itis believed that the liquid acted as a surface lubricant for the balloonand/or a plasticizer for the polymer in the balloon. The liquid may alsohelp to maintain a uniform heat profile along the mold. The parison wasthen placed on the lower half of the mold at a predetermined position,e.g., by placing an end of the parison at an indicator mark, andconnected to the grippers. The horizontally-mounted ball slide allowedthe parison to move horizontally within the mold. The upper half of themold was then lowered onto the lower half. The parison was thenpressurized by introducing N₂ gas into both ends of the parison, andheated to form the balloon. The parison was not drawn axially duringballoon formation. Instead, the parison was allowed to move freely suchthat as the balloon was formed and needed more material to expand, theballoon drew adjacent material toward itself.

The parison was heated to balloon forming temperatures about equal to orgreater than a glass transition temperature (T_(g)) of the parisonmaterial, e.g., about 45-50° C. for Arnitel®. Due to heat loss, e.g.,radiative heat loss to air, the mold was heated to a temperature greaterthan the glass transition temperature of the parison material, e.g.,about 110-115° C., as measured by the thermocouple positioned in thelower half of the mold. The parison was maintained at the balloonforming temperatures for a time sufficient for the balloon to fullyform. Then, the heater was turned off, the mold was force quenched toroom temperature by spraying the mold with water, and the upper half ofthe mold was lifted.

Referring FIG. 10, temperature and pressure profiles during fabricationof the 6-4 balloon are shown. The temperature profile shows an internalparison temperature, as measured by a thermocouple inserted through oneend of the parison and positioned in the center of the parison. At to,the parison was connected to the grippers, and the applied pressure wasrelatively low, e.g., about 37 psi. At t₁, which corresponds to theclosing of the upper half of the mold, the pressure was increased to afirst balloon forming pressure (P₁), here, about 265 psi. Then, at t₂,the heater was turned on and controlled to provide balloon formingtemperatures, here, an internal parison temperature of about 101° C., toform the balloon. About midway through balloon formation (t₃), here,about 35 seconds, the pressure was increased to a second balloon formingpressure (P₂), here, about 273 psi. In the present example, about midwaythrough balloon formation, the temperature, as measured by thethermocouple of the lower half shell, was also increased, e.g., steppedfrom about 123° C. to about 132° C., to maintain the internal parisontemperature of about 101° C. Other manufacturing configurations may notrequire a change in pressure and/or temperature. It is believed that thechanges in pressure and temperature helped to fully expand the parisonto contact the mold and complete balloon formation. The parison was heldat the balloon forming pressures and temperatures for a time sufficientto form the balloon. At a predetermined time (t₄), the temperature wasdecreased, e.g., to room temperature, while the pressure was maintainedat the second balloon forming pressure, P₂. At t₅, the pressure wasdecreased. The balloon was then removed from the mold and the grippers.

To qualitatively characterize the degree of stress in the balloon, theformed balloon was evaluated by directing polychromatic white lightthrough the balloon, and viewing the balloon with a polarizing lens. Insome embodiments, the birefringence pattern generally contains straightand uniform strain lines with minimal taper. FIG. 2A shows thebirefringence pattern of a 6-4 balloon formed by the methods describedabove. In comparison, as shown in FIGS. 2B and 2C, 6-4 balloons in whichaxial movement was restricted during blowing, e.g., by mechanicallyrestricting the ends of the parison, have birefringence patterns thatare non-uniform and not straight, indicating that the balloons haveresidual stress and/or are tapered.

EXAMPLE 3

The following illustrates formation of an 8-4 (i.e., 8 mm O.D. and 4 cmlong) Arnitel® (EM740 from DSM Engineering Plastics) balloon.

A tube of Arnitel® was provided having an inner diameter of about 0.108cm (0.0425 in), an outer diameter of about 0.203 cm (0.080 in), and alength of about 30.48 cm (12 in). The tube was sealed on one end byheating and pressing the end with pliers. Similar to Example 2, the tubewas formed into a parison by axially drawing the open end of the tube atroom temperature. The draw rate was about 0.41 cm/sec (0.16 in/sec); thedraw length was about 12.065 cm (4.75 in); and the maximum internalnitrogen pressure was about 410 psi. A stretched portion about 12.065 cm(4.75 in) long and having an O.D. of about 0.175 cm (0.069 in) wasproduced.

The parison was formed into a balloon using generally the same assemblyand method as described in Examples 1 and 2 (with a different mold).FIG. 11 shows the temperature and pressure profiles during blowing ofthe 8-4 balloon.

The formed balloon was characterized using birefringence toqualitatively evaluate the degree of stress in the balloon.

Other embodiments are within the claims.

1-38. (canceled)
 39. A medical balloon comprising a block copolymerhaving a cylindrically-shaped region exhibiting a birefringence patternof substantially parallel lines before and after an exposure totemperatures of about 32° C. to about 60° C.
 40. The balloon of claim39, wherein the cylindrically-shaped region has a length between about1.5 cm and about 14 cm.
 41. The balloon of claim 39, wherein thecylindrically-shaped region has an outer diameter between about 1 mm andabout 12 mm.
 42. The balloon of claim 39, formed of a polymer havingregions of different hardness.
 43. The balloon of claim 39, wherein theblock copolymer includes hard segments and soft segments.
 44. Theballoon of claim 39, wherein the block copolymer comprises a materialselected from a group consisting of a polyether-ester elastomer, apolyester elastomer, and a polyether block amide.
 45. The balloon ofclaim 39, wherein the block copolymer comprises a polyether-esterelastomer.
 46. The balloon of claim 39, comprising a material selectedfrom a group consisting of a polyester and a polyamide.
 47. The balloonof claim 39, formed of multiple layers.
 48. The balloon of claim 47,formed of multiple coextruded layers having different hardnesses.